Pickling



Patented Nov. 6, 1951 UNlTEDfSTATES ATENT OFFICE 2,574,049 A v I v 7 PICKLING John T. Mayhew, Torontog Ohio, assignor to National Steel Corporation, a corporation of Delaware No Drawing;

This application. is a'continuation-in-part of' my copending application Serial No. 7563315, filed June 24, 1947, nowabandoned.

Iron and steel articles often become coated" with rust or other forms of iron oxide. It is cus' tomary to pickle such articles in sulphuric acid. which attacks the metal beneath the oxide and 1 loosens the oxide to clean and etch the surface of the article. The concentration .of sulphuric acid in a pickling bath may vary over a very Wide range. The particular concentration of acid used depends on a number of variables and usually constitutes from about 2% to about of the solution. The main factor controlling the concentration of the acid is theeconomy of the operation. Acid concentrations or from about 9% to'l4% are often used iii-batch pickling with somewhat higher concentrations beingused when continuously pickling ferrous metal articles such as steel strip. At times, the pickling solution is heated and maintained at an elevated temperature to increase its activity and thereby reduce the time required to effect the desired degree of pickling. As the picklingoperation proceeds, the sulphuric acid and iron react to form ferrous sulphate so that it is customary during the pic kling operation to replenish the. bath with sulphuric acid either periodically or continuously to maintain the desired concentration of free acid. As the ferrous sulphate is formed in the bath, it takes on water to formcopperas. As the ferrous sulphate builds up in the pickling-solution, it passivates the solution so that when the iron content of the bath reaches about i to 6%,

satisfactory pickling is no longer economical or practical. Under some conditions, the pickling solution may be. used until the iron content reaches about 7%. which is the equivalent of about 35% ferrous sulphate calculated as cop- Sulphuric acid pickling contain one '40 activity of the solution is so greatly reduced that v Application November 12; 1948; Serial N0. 59,759

25 clai s. (01. 252-142) '2 j or more inhibitors and wetting agents herein, the term sulphuric acid p'ckling bath? includes a pickling bath containing sulphuric acid and in which copperas is the iron salt produced as a result of the pickling action. The

primary purpose of an inhibitor is to reduce the rate of attack on the base metal by thesulphur-ic acid without or while only slightly reducing the:

rate of removal of the oxide from the surface.v The addition of a wetting agent assists the acid to penetrate theoxide' and thus promotes efii cient pickling The effect of various acid concentrationsiin hibitors and wettingagentsj and the effect of various temperatures on the pickling bath are.

well known to the art and are discussed in the following technical books:

1. Tinplate, Hoare and Hedges, Great Britain by Butler and Tanner Ltd., 1945; pages 49 to 65.

2. Ferrous Metallurgy, Teichert, vol. 2, 1944',

pages 272 to 279,

3. The Making, Shaping and Treating of: Stee C; B. Francis, 5th ed.,- pages 1l18 to 1192..

There has been a long felt need of counteract ing or reducing the passivating action of the fet 'rous sulphate in sulphuricacid pickling solutions so as to increase the useful life of thepickling bath. If the quantity of ferrous sulphate which the pickling solution can tolerate could. be

increased; then the amount of pickling which; could be effected without dumping the pickling; solution would be increased. This in turn would.

reduce the cost of the, pickling operation by reducing the amount of wastedaoid per ton of metal treated and by reducing the time lost in changing the pickling bath. Reducing theamount of waste pickle liquor would in turn reduce the. problems involved in disposingof this liquor which often isan expensive operation. Length 1 ening the pickling period in an attempt to 'coum teract the passivating action of the ferroussuh.

In some plants, the

phate .is; uneconomical. pickling solution is discarded when containing.

only a very small amount of ferrous sulphate be cause increasing the pickling period is so highly.

undesirable.

which are subsequently to be galvanized; it is desirable *to'etch the surface of the sheets so that they do not present a mirror-like appearance;

Such sheets should be uniformly etched and free of so-called pickle patches which are relatively v.bright areas that-are not sufficientlvetched to printed remove the mirror-like surface. This etching of the surface is not entirely a problem of removing metal. For some reason, some surface areas are apparently more reactive than others. Pickle patches are particularly undesirable when the sheets are to be galvanized as they reduce the adherence of the galvanizing metal to the base metal.

I have discovered that the addition to sulphuric acid pickling baths of a very small quantity of a compound selected from the group consisting of titanium tetrachloride, titanium trichloride and mixtures thereof increases the tolerance of the bath for ferrous sulphate and also increases the etching power of the bath so as to eliminate or reduce the occurrence of pickle patches. The addition of one or more of these compounds also tends to increase the activity of the pickling bath so that a somewhat shorter period is required to efiect pickling. Increasing the etching power or activity of the pickling bath reduces or eliminates the occurrence of pickle patches and permits greater etching of the steel where desirable. Increasing the tolerance of the bath for ferrous sulphate increases the life of the bath and the amount of steel which may be pickled before the bath is dumped and reduces the cost of the sulphuric acid by reducing the amount of acid that is discarded. In commercial production, I have used from one to two pints of the chloride for each 1600 gallons of pickling solution and have operated these pickling baths when they contained more than 60% copperas.

The quantity of titanium tetrachloride or titanium trichloride or mixtures thereof which is added to the pickling bath depends upon a number of variables. The effect of the chloride is Somewhat proportional to the amount added. Very small quantities of the chloride produce a noticeable effect on the action of the pickling bath. Quantities as small or less than one part of chloride in 12,000 parts of pickling solution produce a noticeable effect on the etching power of the pickling solution, and much smaller quantities of chloride increase the tolerance of the bath for copperas. Both of the chlorides are expensive, and it is undesirable to use more than isnecessary. The use of more than about 1% by weight of chloride does not cause a material increase in the etching power of the bath or in the tolerance of the bath for copperas over above that obtained with 1% chloride. The addition of from .01 to 1.0% by weight of the chloride to the pickling bath will reduce or eliminate the occurrence of pickle patches and will increase the etching effect on the base metal beneath the oxide coating. the choride, such as .008% by weight, will improve the etch, but the very small quantities of chloride which increase the tolerance of the pickling solution for copperas do not always increase the etching power of the pickling bath. As a result, when it is necessary to increase the Somewhat smaller amounts of 7 found that a pickling solution containing a very small amount of one of these chlorides not only is not passivated by the presence of the copperas, as is the ordinary sulphuric acid pickling solution,- but, that as the copperas increases, the etching power of the pickling bath tends to increase and is greater than the etching power of the ordinary pickling solution. It has been possible to use these hot pickle solutions containing small amounts of chloride until they contain so etch, it is desirable to use at least .008% by much copperas that when they are allowed to cool the copperas forms a solid body of approximately the same size as the body of pickling solution.

It is possible that the titanium tetrachloride and the titanium trichloride may enter into some kind of a chemical reaction so that these chlorides are not present in the pickling solution as such but are present in a diiferent form or forms. Accordingly, when the expression added is used in connection with these chlorides, the term is to indicate that the chloride is added as such but may be present in a different form in the pickling bath. It has been found that titanium tetrachlo ride and titanium trichloride are substantially equally effective andmay be used in the same quantities to produce the same results. Accordingly, when a particular quantity of, for example, titanium tetrachloride is mentioned, it is to be understood that the same quantity of titanium trichloride or the same quantity of mixtures of these chlorides could be substituted.

The following examples illustrate my invention, but are not intended to limit my invention to the particular proportions or quantities set forth, except as clearly set forth:

Bath at time of Dis- Bath at Start carding Example 4 Temp.

No. oglrfigtlltgl Per C81-It o F. Per Cent Per Cent g ggfik flgfi mso. resonmo solution 1% pints 1 5 180 7. 6 46 1% pints- 12. 5 8. 3 49 1pint 9 170 8.9 43

Steel sheets were batch pickled on a commercial scale in the bath of each example. Sulphuric acid was added periodically in the customary manner to maintain the acid content between about 9% to 14% by weight. Each of these baths was quite active, and the degree of etching was appreciably greater than that obtained with a similar hath not containing titanium tetrachloride over the same period. For each of these copperas content caused pickle patches, which are areas not cleaned or etched. The b'aths of Examples 1 to 3 were discardedwhenthe copperas content was as specified because :the =baths were contaminated with .d irtand oil and werenotidis-i' copperas content prolongs the life of .the -bath. and reduces the amountof acid discarded. "This. in turn reduces the cost and the problems .of dis.-

posal. If titanium tetrachloride had not been added to the bath of Example 1, it would have been necessary to discard the bath when it con- Thus, 'in' order tained less than 30% copperas. to effect the same amount of etching, it would have been necessary to use more than .onesnor! mal bath which would have multiplied thelacid loss. Increasing the activity oretchingpower of the bath produces satisfactory etching more readily, and, in at least some instances, produces results not obtainable with normal sulphuric acid pickling baths.

Similar commercial baths were made and tested using titanium trichloride with thev same improvements being noted. Mixtures of the tetra chloride and trichloride were tested. I

A series of panels were pickled in sulphuric acid solutions containing 8% by weight of the sulphuric acid. Titanium tetrachloride was added to some of the solutions and titanium trichloride was added to some of the other solutions. The amount of chlorideadded was equal to .one'part in 12,000 parts by weight of pickling solution. A

third of the pickling solutions did not contain any titanium chloride. The pickling period was varied from two minutes to minutes, and the amples were weighedbe-fcr and after bein pickled in the three types of solutions, The .loss in weight for each sample was calculated and the loss in weight of those samples pickled in the solutions containing titanium chloride was compared to the loss in weight of the s -ples pickled in the sulphuric acid alone. There was no appreciable difference in the weight lost. I All of the samples were dried and inspected. It was possible to pick out those samples which had been pickled in solutions containing one or the other of the chlorides by the fact that they had a lower degree of surface reflection indicating a greater etching power of the solutions containing the chloride.

A second series of panels were pickled in a similar series of pickling solutions, some of which contained 1% by weight of titanium tetrachloride and others contained 1% by weight of titanium trichloride. Control panels were pickled in similar sulphuric acid pickling solutions without any chloride for similar periods. All of the samples were tested in an Olsen cup tester to determine the depth of the cup at fracture. This test showed that neither the tetrachloride nor the trichloride had any effect in reducing hydrogen embrittlement of the panels.

Titanium tetrachloride rapidly breaks down in the presence of moisture, and, in fact, its greatest use has been in the production of fogs for the protection of troops. When titanium tetrachloride is used, it should be added to the bath after at least some of the sulphuric acid has been added. Not all the acid needs to be present when the tetrachloride is present but at least suflicient sulphuric acid must be added to make the bath acidic to prevent breakdown of the titanium tetrachloride. 1f desirable, the remainder of the acid or additional acid may be added*after.- the tetrachloride or a concentrated;

sulphuric acid-titanium tetrachloride solution maybe mad 'up and used as needed to make up pickling solutions.

Iclaim;

1. A sulphuric acid pickling bath containing an added compound selected from the group consisting .of titanium tetrachloride, titanium trichloride and mixtures thereof, said compound being present in an amount not greater than 1% by weight of the bath.

A sulphuric acid pickling bath containing from 2% to.20% by weight of sulphuric acid and an added compound selected from the group consisting .of titanium tetrachloride, titanium trichloride and mixtures thereof, said compound being present in an amount not greater than 1% by weight of the bath..

3. .Azsulphuric acid pickling bath containing.

anadded compound selected from the group consisting of titanium tetrachloride, titanium trichloride and mixtures thereof, said compound being present in an amount between .008% and 1% by weight of the bath.

.4. A sulphuric acid pickling bath containing an added compound selected from the group con-.- sisting of titanium tetrachloride, titanium tri-.- chloride and mixtures thereof, said compound being present in an amount of from one pint per 1600 gallons of bath to 1% by weight of the bath.

' 5. A sulphuric acid pickling bath containing an added compound selected from the group consisting'of titanium tetrachloride, titanium trichloride and mixtures thereof, said compound being present in an amount of about one and one-half pints per 1600 gallons of bath.

6. A sulphuric acid pickling bath containing added titanium tetrachloride in an amount not greater than 1% by weight of the bath.

-7. A sulphuric .acid picking bath containing from12% to 20% byfiweight of sulphuric acid and added titanium tetrachloride in an amount not added titanium tetrachloride in an amount offrom .008% to 1 by weight of the bath.

9. A sulphuric acid pickling bath containing added titanium trichloride in an amount not greater than 1 by weight of the bath.

10. A sulphuric acid pickling bath containing from 2% to 20% by weight of sulphuric acid and added titanium trichloride in an amount not greater than 1 by weight of the bath.

'11. A sulphuric acid pickling bath containing added titanium trichloride in an amount of from .008% to 1% by weight of the bath.

12. In the process of pickling ferrous metal articles, the step of pickling such an article in a sulphuric acid pickling bath containing an added compound selected from the group consisting of titanium tetrachloride, titanium trichloride and mixtures thereof, said compound being present in an amount not greater than 1% by weight of the bath.

13. In the process of pickling ferrous metal articles, the step of pickling such an article in a sulphuric acid pickling bath containing from 2% to 20 by weight of sulphuric acid and an added compound selected from the group consisting of titanium tetrachloride, titanium trichloride and mixtures thereof, said compound being present in an amount not greater than 1% by weight of the bath.

14. In the process of pickling ferrous metal articles, the step of pickling such an article in a sulphuric acid pickling bath containing an'added compound selected from the group consisting of titanium tetrachloride, titanium trichloride'and mixtures thereof, said compound being present articles, the step of pickling such an article in abath containing titanium tetrachloride in an amount not greater than 1% by weight of the bath.

16. In the process of picklingferrous metal articles, the step of pickling such an article in a sulphuric acid pickling bath containing from 2% to 20% by weight of sulphuric acid and titanium tetrachloride in an amount of from .008% to 1% by weight of the bath.

17. In the process of pickling ferrous metal articles, the step of pickling such an article in a bath containing titanium trichloride amount not greater than 1% by weight of the bath. I V

18. In the process of pickling ferrous metal articles, the step of pickling such an article in a sulphuric acid pickling bath containing from 2% to 20% by weight of sulphuric acid and titanium trichloride in an amount of from .008% to 1% by weight of the bath.

19. In the process of pickling ferrous metal articles, the step of pickling such an article in a sulphuric acid pickling bath containing an added compound selected from the group consisting of titanium tetrachloride, titanium trichloride and mixtures thereof, said compound being present in an amount not greater than 1% by weight of the bath, and containing more than 35% by weight of copperas.

20. In the process of pickling ferrous metal articles, the step of pickling such an article in a sulphuric pickling'bathcontaining from 2% to 20% by weight of sulphuric acid, containing an added compound selected from the group consisting of titanium tetrachloride, titanium trichloride and mixtures hereof, said compound being present in an amount not greater than 1% in an.

by weight of the bath, and containing morethan 35% by weight of copperas. "'1

.21. A sulphuric. acid pickling bath containing titanium tetrachloride in an amount equal to about 1%. pints per 1600 gallons of pickling bath.

22.An aqueous ferrous metal pickling bath containing pickling acid consisting of sulphuric acid andcontaining an added material selected from the group consisting of titanium tetrachloride, titanium trchloride and mixtures thereof, said material being present in an amount of from 1 pint per 1600 gallons of bath to 1% by weigh of the bath.

23. An aqueous ferrous .metal pick1ing bath containing pickling acid consisting of sulphuric acid and containing added titanium tetrachloride in an amount of from 1 pint per 1600 gallons of bath to 1% by weight of the bath.

24. The process of pickling a ferrous metal article in a pickling bath containing pickling acid consisting of sulphuric acid and containing an added material selected from the group consisting of titanium tetrachloride, titanium trichloride and mixtures thereof, said material being present in an amount of from 1 pint per 1600 gallons of bath to 1% by weight of the bath.

25. The process of pickling a ferrous metal article in a pickling bath containing pickling acid consisting of sulphuric acid and containing titanium tetrachloride in an amount of from 1 pint per 1600 gallons of bath to 1% by weight of the bath.

JOHN T. MAYI-IEW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 2,450,861 Robinson Oct. 5, 1948 

1. A SULPHURIC ACID PICKLING BATH CONTAINING AN ADDED COMPOUND SELECTED FROM THE GROUP CONSISTING OF TITANIUM TETRACHLORIDE, TITANIUM TRICHLORIDE AND MIXTURES THEREOF, SAID COMPOUND BEING PRESENT IN AN AMOUNT NOT GREATER THAN 1% BY WEIGHT OF THE BATH. 